In situ observation of solidification of undercooled hypoeutectic Ni-Ni3B alloy melt

Solidification of undercooled Ni-3.3 wt% B alloy melt was investigated by glass fluxing. If Delta T-e, < 140 +/- 10 K, two recalescences appear, indicating that stable eutectic reaction occurs; if Delta T-e >= 140 +/- 10 K, three recalescences can be observed, indicating that metastable eutectic reaction occurs. Analysis indicates that the phase fractions of the as-solidified structure can be predicted by the recalescence delay times in the cooling curves. High-speed video images show that the solidification interface of primary solidification changes from single dendritic shape to spherical shape with increasing Delta T-p; the interface of eutectic solidification changes from many small "dendrites" to a single large one with increasing Delta T-e; the interface of residual liquid solidification changes from many small rings to a single large one with increasing Delta T-r. The growth velocity of eutectic solidification suggests a coupled growth at small and moderate undercoolings and decoupled growth at large undercooling, whereas that of residual liquid solidification cannot be interpreted by the available models.